1. Field of the Invention
The present invention relates in general to a control device for a vehicular drive system including an engine and a differential mechanism which functions as an electrically controlled differential device having a differential function, and more particularly to techniques for controlling the vehicular drive system upon stopping of the engine.
2. Description of Related Art
There is known a drive system for a vehicle, which includes a differential mechanism arranged to distribute an output of an engine to a first electric motor and an output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and drive wheels of the vehicle. Examples of this type of vehicular drive system include drive systems for a hybrid vehicle as disclosed in JP-10-306739A, JP-2000-346187A and JP-2000-2327A, typically in JP-10-306739A. In these hybrid vehicle drive systems, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of the drive force generated by the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the rest of the drive force is electrically transmitted from the first electric motor to the second electric motor, through an electric path therebetween, so that the differential mechanism functions as a transmission such as an electrically controlled continuously variable transmission the speed ratio of which is electrically variable, thereby making it possible to drive the vehicle under the control of a control device, with the engine kept in an optimum operating state with an improved fuel economy.
A continuously variable transmission is generally known as a power transmitting mechanism suitable for improving the fuel economy of a vehicle, while on the other hand a gear type transmission device such as a step-variable automatic transmission is known as a power transmitting mechanism suitable for improving the power transmitting efficiency. However, there is not known any power transmitting mechanism that is suitable for improving both of the fuel economy and the power transmitting efficiency. The hybrid vehicle drive system disclosed in JP-10-306739A, for example, has an electric path through which an electric energy is transmitted from the first electric motor to the second electric motor, that is, a power transmitting path through which a portion of the vehicle drive force which has been converted from a mechanical energy into an electric energy is transmitted. This drive system requires the first electric motor to be large-sized with an increase of the required output of the engine, so that the second electric motor operated with the electric energy supplied from the first electric motor is also required to be large-sized, whereby the drive system tends to be unfavorably large-sized. The same drive system also suffers from a risk of deterioration of the fuel economy during a high-speed running of the vehicle, for example, due to conversion of a portion of the mechanical energy produced by the engine into an electric energy, which is subsequently converted into a mechanical energy to be transmitted to the drive wheels of the vehicle. A similar problem is encountered in a vehicular drive system wherein the differential mechanism is used as a transmission the speed ratio of which is electrically variable, for instance, as a continuously variable transmission which is a so-called “electrically controlled CVT”.
A vehicle is generally subject to various kinds of vibration. An engine of the vehicle, which is one of vibration sources, has a variation of its torque during its operation, which causes torsional vibration of a power transmitting system (vehicular drive system), which is amplified by a resonance phenomenon and transmitted to the body of the vehicle through a damping device such as engine mounts. As well known in the art, the resonance phenomenon may take place in a specific speed range of the engine called “resonance speed range”, for example, a speed range the upper limit of which is not higher than an idling speed of the engine. When a condition for stopping an operation of the engine is satisfied, a fuel supply to the engine is stopped so that the engine speed is lowered to zero. In this process of lowering of the engine speed upon stopping of the engine, the engine speed falls into the resonance speed range in which the resonance phenomenon may takes place. In the hybrid vehicle drive system as disclosed in JP-10-306739A, the first electric motor is operated to lower the engine speed to zero at a high rate through the resonance speed range, for thereby reducing the risk of occurrence of the resonance phenomenon.
It is desirable to further reduce the risk of occurrence of the resonance phenomenon upon stopping of the engine even in the hybrid vehicle drive system arranged to reduce the risk in the manner described above.